This invention relates generally to an improved material for a permanent magnet, and more particularly a composition of samarium-transition metal forming a single phase in the molar ratio of about 2 to 17.
The rare-earth metals and cobalt form various intermetallic compounds such as RCo.sub.13, R.sub.2 Co.sub.17, RCo.sub.5, R.sub.5 Co.sub.19, R.sub.2 Co.sub.9, RCo.sub.3, RCo.sub.2, R.sub.2 Co.sub.3, R.sub.4 Co.sub.3, R.sub.24 Co.sub.17, R.sub.9 Co.sub.4 and R.sub.3 Co. RCo.sub.5 and R.sub.2 Co.sub.17 have been used as a material for permanent magnet. Specifically, the RCo.sub.5 type magnets are used for industrial purposes and the demand for these magnets is increasing with time. A samarium alloy, SmCo.sub.5 is an example of the RCo.sub.5 -type magnet which provides a greater energy product than the conventional alnico, ferrite and plutonium-cobalt magnets. Generally speaking, R.sub.2 Co.sub.17 has a higher saturation magnetization (Ms) than the RCo.sub.5 alloy so that one would expect the R.sub.2 Co.sub.17 to be a suitable material for a magnet. However, the R.sub.2 Co.sub.17 has a lower coercive force (Hc). Therefore, it was once considered that there was no prospect of putting this latter material into practical use. It has since been learned that the coersive force may be increased by adding an appropriate amount of copper, thus drawing renewed attention to this material.
R.sub.2 Co.sub.17 and R.sub.2 Fe.sub.17 form a psedobinary system which may be represented by the formula: R.sub.2 (Co.sub.1 -x Fe.sub.x).sub.17. The saturation magnetization (Ms) increases corresponding to an increase in the value of x to the extent of about 0.6 when the uniaxial anisotropy is maintained. The anisotropic magnetic field (Ha), which is considered as the upper limit of the coercive force is 65 KOe when x equals 0, decreases when x equals 0.2 and is about 20 KOe when x equals 0.5. However, it has been found that when the iron is added to the Sm.sub.2 (Co Cu).sub.17 in amounts even less than 10 weight percent, the iron causes considerable deterioration of the coercive force. Thus, the advantage that the saturation magnetization is increased due to the presence of iron, has not been efficiently used.
In contrast, when copper is added to the alloy the saturation magnetization (Ms) is decreased considerably, even when the coercive force is increased. It has been found that when cobalt in the Sm.sub.2 (Co Cu).sub.17 is replaced by copper, the saturation magnetization decreases 12% with a 8 weight percent replacement of copper and Ms decreases 25% with a 16 weight percent replacement by copper. The decrease in coercive force caused by the replacement with iron and the decrease of the saturation magnetization caused by the replacement of copper may be improved by a small addition of the other element. If the copper replacement is maintained below 8 weight percent and the iron replacement is above 15 weight percent, then intrinsic coercive force (iHc) of about 6 or more KOe may be obtained.
In a magnetic material for a permanent magnet including zirconium, the ratio of the samarium to the transition metal (hereinafter referred to as "the TM ratio") is no longer 2 to 17, but is approximately 1 to 7. In the range near the TM ratio of 1 to 7, metal of the 1-5 phase having the TM ratio of 1 to 5, and metal of the 2-17 phase having the TM ratio of 2 to 17 coexist.
In an evaluation between the SmCo.sub.5, which forms the fundamental 1-5 phase, and the Sm.sub.2 Co.sub.17 which forms the fundamental 2-17 phase, the Ms of the SmCo.sub.5 is as low as 10.0 Kg. When iron replaces the cobalt in the Sm.sub.2 Co.sub.17, saturation magnetization is increased. In contrast, in the SmCo.sub.5 material, it has been found that Ms is barely effected by the replacement with iron. Additionally, the following facts have been determined from X-ray analysis. Namely, that the solid solubility of copper and the 1-5 phase is greater than that in the 2-17 phase. Specifically, more copper disolves into the 1-5 phase in the range wherein the two phases coexist. Thus, in the range where the TM ratio of 1 to 7 the 1-5 phase having low saturation magnetization exists in large amounts. In addition to the fact that the 1-5 phase has a large capacity for copper which reduces the Ms, zirconium present in the 1-5 phase traps the iron and forms a new composition. Therefore, it has been difficult to obtain a high performance magnetic material. Accordingly, it would be desirable to provide a high performance permanent magnet overcoming the shortcomings encountered with the prior art compositions.